Tumble-finishing process and media therefor



June B3, w67 T. J. LESTER TUMBLE-FINISHING PROCESS AND MEDIA THEREFOR Filed June 9, 1964 This invention relates to a method of abrading surface irregularities from hard-surface articles, such as small metallic castings, stampings, or forgings by continuous or batch treatments in tumbling or vibrating mills.

The term abrading is used herein in its broad sense and is intended to include substantially any of the finishing treatments performed in a barrel or vibratory finisher. These operations include definning, deburring, grinding, descaling, polishing, burnishing and cleaning of articles which may comprise, for example, any one of many metals, rubber, plastic, and ceramics.

In general, such treatments are dependent upon a media, or solid material, with which the work articles are mixed. The media is the principal agent toeffect the desired finishing treatment and may be a dry material, or provided in a wet mixture, which comprises also a vehicle or liquid, such as Water. The media, because it determines the nature of an abrasive action on the work article, is selected in view of the relative hardnesses of the articles and the media, the nature of the irregularities to he removed from the article, the fineness of finish desired, the shape of articles, and' other factors.

Among media in common use are chips which include chunks, chips, or pebbles of natural stone, or artificial stone-like materials, such as particles of aluminum oxide or carbides mixed with a tough bonding ceramic. Soft metal forms used in combination with particle abrasives of various fineness and hardnesses of materials, such as pumice, emery and quartz sand, unfused alumina, are useful for grinding operations. Rubber and nylon shapes, of which the resinous composition is mixed during manufacturing with an abrasive of any one of many particle sizes, are availa'bl-e as media, eg., for grinding, deburring, and polishing. Hardwood pegs, balls and other forms are useful as media for finishing work articles of plastic. Sawdust, ground corn cobs, husks, are typical of media used as a cushioning agent in admixture with other harder materials, and in other instances for cleaning and drying parts after an abrasive treatment. Ceramic forms, usually triangular in shape, are used extensively for definning and deburring non-ferrous castings. These media aremerely exemplary of the various materials available.

There are some aspects of barrel or vibratory finishing that invite improvement. In all such instances wherein the media comprise shapes as distinguished from sandlike particles, the ymedia are subject to attrition through use and then inevitable discard. Replenishment of media stocks is a substantial item of expense. In the use of shaped media in the finish-treating of articles which have pockets which entrap the media, subsequent cleaning treatments may fail with complete certainty to remove media elements and thus render necessary, at the least, inspection of finished articles, and perhaps manual removal of elements from the articles. Another factor which contributes to the cost of finishing with shaped media is the labor and extra equipment entailed in removing the load from the finishing equipment and separating the work articles from the shaped media. This operation is somewhat simpler when the work articles are mixed merely with liquid and' a ne particle abrasive material. Separation of shaped media and the work requires the use of special machinery when not accomplished manually.

An essential object of this invention is to provide a rllll l method of preparing media for use in tumbling barrels, vibratory equipment, or other equipment for producing relative movement between work articles and finishing media.

Another important object is to provide a new method of finish-abrading batches or continuous processions of articles requiring surface finishing including the preparation of suitable media for the desired finishing treatment to be accomplished.

It is a further object to provide a new type of media for surface-finishing batches or continuous processions of articles made, for example, of metal or plastic.

It is also an object, in providing new finishing media and a process including the preparation and the use of the media, to enable reconstituting or reformation of the media after use in order that the media may, in effect, be reused repeatedly to thereby achieve economies in finishing operations.

Still another object is to simplify the removal of media from `batches of finished articles, especially, those types that are prone to retain and trap fragments of shaped media. An object ancillary to such simplification is to eliminate special sorting machinery.

Very briefly, the invention achieves the above objects and others to be apparent herein through the preparation and use of a frozen media. In the simplest instance, the medium may be simply shaped pieces of a frozen liquid but in most instances the medium will take the form of predetermined shapes comprising a mixture of frozen liquid, usually water, and a solid abrasive held within the frozen liquid with the choice of the abrasive depending upon particle size, the hardness of the particles desired, the size, shape, and hardness of the work articles, and the finish desired thereon.

The process with respect to preparation of the media is the formation thereof by freezing it into a desired pellet size, or freezing into larger masses and subdividing it. The introduction of the frozen media results in modifications of conventional finishing treatments. For example, the frozen media may be continuously fed into the finishing machine and melted media components removed or, in another instance, a batch vessel of a finishing machine is jacketed to receive a refrigerant whereby melting of the media is controlled. Both continuous feeding and refrigeration may be concurrently practiced.

In large scale continuous finishing operations, the components of the media are continuously removed from the finishing machine, refrozen into media, and recycled. In batch operation, the finishing machine load at the end of the tumbling or vibrating period is heated, if necessary, to reduce all lumps of media to a melted fiowable condition, and the work batch is separated and cleaned in situ, i.e., without prior removal from the finishing machine. In the alternative, the charge of work articles and media are dumped from a batch machine vupon a screen whereupon separation of articles and media is achieved by flushing with Water. This treatment may be preceded by a warm bath to melt such media. entrapped within the articles.

In the drawing, with respect to which the invention is described below in more detail:

FIG. 1 is a process diagram for a batch process of surface finishing of articles.

FIG. 2 is a process diagram for a continuous process of surface finishing of articles.

FIG. 3 is a diagrammatic view of a barrel finisher.

FIG. 4 is a diagrammatic View of a vibratory finishing machine.

Referring to FIGS. 1 and 2, it will be apparent that the rst major step in practicing this invention is the preparation of the media. As mentioned above, such preparation may involve only the making of ice cubes or pellets through the operation of a commercial industrial-type icecube making machine. Where the temperature within the finishing machine is controlled by refrigeration, ice shapes are a useful medium for polishing many types of articles, such as those made of plastic and wood. Temperature and drainage are regulated as required to prevent the presence of liquid in the finishing machine to an extent impeding the finishing action of the ice media. The shape of ice bodies made is essentially the matter of providing suitable water-receiving molds of proper cavity. A machine of this type is used also for pellets, cubes or other shapes, comprising water and an abrasive such as quartz sand, chips of mineral material, emery, aluminum oxide, sawdust, and other solid but particulate grinding or polishing materials. In general, media in accordance with the invention will comprise solid material of higher density than water and difficult to suspend therein. Hence, the amount of water occurring in the solid component of the media of greater density than water will be merely that amount which fills the interstices between the particles of solid component.

One of the great advantages of this invention is that an infinite variety of media are possible through great variety of particle sizes available, and variety of materials used either singly, or in combinations of more than one solid component in admixture with the liquid vehicle. For example, particle sizes may vary from that of very fine dust, suitable for producing high luster finishes, to pea and nut sizes or larger.

The frozen pellets, when they contain merely interstitial water and particularly when the particle size of abrasive component is small, are quite resistant to fracture and tend merely to wear away. The rate of attrition of the pellets is dependent to a great extent on the degree of cooling obtained by precooling the work articles and refrigerating the finishing machine. A high-density medium is obtained which is also highly resistant to breakdown either by wear or fracture at sub-freezing temperatures when the solid component comprises a variety of particle sizes graduated, in the manner of skillful concrete mixing, to dispose ever smaller particles within the interstices of the larger particles. In such a medium, attrition on the whole pellet is influenced greatly by the wear on the larger particles contained in its outer surface.

With conventional media, such as chips of stone or synthetic stone-like materials, the finishing machine load will generally include water which substantially submerges, or more than submerges, the other components of the load. In practicing the present invention, water as a liquid may also be employed as a component of the machine load. However, as the media herein disclosed melts lreadily in water, refrigeration of the water of the finishing vessel may be used to maintain the load at a temperature near the freezing point of water to control melting of the media. Also, refrigeration may be carried to any point desired in solidifying the water component of the load. For example, it may be desired to obtain a slushy condition which will cushion the contact of the work articles with each other or with the media in the manner provided by sawdust or corn cob materials. It may be further desirable to reproduce dry conditions by refrigerating all water of the load to a particulate icy state as the motion of the finishing machine is continued. The icy material then acts as a cushioning agent in the presence of the frozen finishing medium in pellet form. In still other finishing treatments, water will not be added to the load, and melted media will be allowed to accumulate to the extent desired, or prevented from melting, by controlling the temperature of the work articles in pre-finishing storage and the temperature of the finishing vessel.

As an example, stamped steel hinge parts of complex decorative design and general dimensions of four inches by two inches are deburred and cleaned in preparation for plating in accordance with the process diagram of FIG. l. As a first step, pellets of one-half inch medium dimension are prepared in a standard industrial-tipe ice-cube machine from aluminum oxide particles mixed with sufficient water to fill the interstices. The frozen pellets along with the charge of hinge parts are placed in the load-receiving vessel of a conventional vibratory finishing machine 2, indicated diagrammatically in FIG. 4, in the ratio of three units by volume of :pellets to one unit of parts. The machine load is completed by filling with water cooled to approximately 35 F. or lower. A cooling medium is circulated through the jackets to maintain the temperature of the load just above freezing temperature. The machine is operated two hours to accomplish the finishing step of FIG. l, during which time the pellets become substantially melted. Thereafter, the melted media are removed through the duct 4 by opening the valve 3 and returned to the pellet-making machine or a storage facility therefor. Particulate abrasive components of the media remaining in the charge of hinge parts after draining are flushed therefrom by a hot water jet accompanied by mild motion of the finishing machine. Any heretofore unmelted media will be melted by the water treatment. All abrasive component is received in a settling basin for recycling through the pellet-freezing step. Thereafter, the hinge parts are removed while hot to a centrifugal dryer which is operated until the hinge parts are dry.

As another example, the process of FIG. 2 may be practiced in finishing malleable iron rocker valve arms for combustion engines in a continuous manner with respect to both the feeding and discharging of the work pieces and feeding and discharging of the finishing media. Freezing of one-quarter-inch and three-quarter-inch pellets is conducted in an industrial type ice cube machine on a continuous basis and fed in equal parts into the rubberlined finishing section of an elongate tumbling barrel of a conventional continuous barrel finisher 7. The finisher has a conventional perforated rinsing section capable of receiving and discharging liquid, and a perforated drying section, through which streams of air may be passed, in series relation with the finishing section 8 shown diagrammatically in FIG. 3. The frozen pellets which consist of silicon carbide grains filled with interstitial water are fed in a ratio of three iparts by volume to one part of the rocker arms. The feed of the media and the rocker arms is adjusted for 11/2-hour passage through the finishing section. Media is discharged into the rinser in fully melted condition. Hot water spray in the rinsing section with continued tumbling of the rocker arms removes all carbide material. The carbide material is continuously returned to the pellet-making machine for refreezing and recycling through the finishing operation. In the drying section, the rocker arms are subjected to a stream of heated air directed into the drying section. Finished and dried rocker arms are continuously discharged from the latter section into a hopper or onto a conveyor as desired.

While the foregoing examples disclose water as the liquid component of the finishing media, the invention embraces, in a practical way, all materials which may be frozen into a hard matrix for abrasive particles and form pellets which are fracture-resistant to the tumbling action of finishing machines herein referred to. The term tumble-finishing is used herein generically to refer to any finishing processes performed in barrel, vibratory or other type of equipment used to induce relative motion between a group of work articles and media. Such matrix-forming materials melt at relatively low, or ordinary, temperatures. The terms low-melting or ordinary temperatures are intended to include those temperatures up to the boiling point of water at the upper extreme, and 0 F., for example, at the lower extreme so as to enable the frozen media of this invention to be handled under practical shop conditions without resorting to complicated, non-conventional, and ultra-expensive equipment. It is understood, of course, that minor departures from this range of temperatures are within the concept of this invention. Within the temperature range just indicated, low-melting fusible eutectic and near-eutectic metal alloys from the group comprising metals such as bismuth, tin, cadmium, indium and lead are known which provide metallic liquids at temperatures ranging from 110 F. to 212 F.

The fusible alloys are soft metals with a hardness on the order of that of lead or less. Thus, pellets thereof containing no abrasive material may be made a component of a finishing machine loa-d to accomplish a finishing treatment, e.g., on brass or plastic pieces, and on many other materials which are softer than ferrous materials. These all-metal pellets may be prepared by melting the alloy and pouring it into any suitable pellet-forming, molding equipment. The cutting power of such pellets is considerably increased if abrasive particles are included in the load with the all-metal pellets. The abrasive particles enter the finishing action by being caught in the soft surface of the pellets and are scraped or rubbed across the surface of the work articles. While the particles remain to a large extent unattached and free, the surfaces of the soft metal pellets become imbedded with the abrasive material and thus become more efficient as a finishing media.

The fusible alloys may be mixed with abrasive materials to supply a matrix in which the abrasive particles are supported. On account of the substantially greater specific gravity of the fusible alloys than the abrasive particle materials commonly employed in the tumble-finishing trade, preparation of composite fpellets containing both ingredients is somewhat more difficult than when the abrasive material is mixed with a liquid, such as water, having a lower specific gravity. It is expedient, for example, to provide the alloy in fine particle form, such as in the form of fine shot or chips, wherein it is mechanically mixed in a ldesired ratio with abrasive particles. The mixture is then placed in pellet-sized receptacles and gently heated to a temperature slightly under the melting point of the alloy to obtain a sintering or coalescent effect without actually liquifying the mixture. The coalescing may be implemented by mechanical pressure. When cooled, the resulting pellets are extremely effective as finishing media. The type of use for which such media is employed will, of course, depend on the nature of the material to be finished and the hardness and the fineness of the abrasive material contained in the pellets.

The advantage in the use of low-melting metals and metal alloys is that the media including unmelted metal may be completely flushed from the work particles by steam or hot water jets. The metallic material and the abrasive component of the media, because of their high density relative to water are readily reclaimable and capable of recycling and repeated use as described with respect to water-abrasive particle mixtures.

The art of tumble-finishing includes the use of many additives or compounds which, in general, are carried in the water which fills the interstices of the load in a great majority of finishing treatments. Among such additives are detergents, water softeners, lubricants, descaling chemicals, alkaline cleaners, soaps, polishing creams and others. This invention includes the addition of any additive which is miscible with the unfrozen vehicle or mixes therewith in a homogenous manner. Introduction of additives into the pellets may be practiced insofar as substantial elevation of melting point or reduction in fracture resistance of the pellets does not occur. In general, the storing of additives in the frozen pellets and t-he release thereof into the finishing load does not introduce the additive into the finishing load in the concentration desired at the beginning and throughout a batch treatmet. However, in continuous processes, wherein media is continually added, and in the instance wherein gradual replenishment of the additive to a batch load is required, inclusion of an additive within the frozen pellets will be found advantageous in some cases.

The invention described above brings to the art of tumble-finishing a great variety of apparently new finishing media which may flexibly fill special requirements of the trade. The invention also provides a process useful in various modifications, of which some are suggested above, which includes the preparation of finishing media at the site of use, the actual finishing treatment, the reconstituted media to the finishing stage of the process. The economic advantages of the invention are attainable particularly in large-scale commercial finishing operations, and with work pieces of irregular, pocketed design.

What is claimed is:

1. A method of removing surface irregularities from hard-surface articles comprising:

(A) freezing portions of a material which comprises a liquid which freezes into a rigid solid at temperatures in the range of 0 F. to 212 F. into pellets of a desired size and including also With said material, a particulate abrasive material of greater hardness to form surface-finishing media;

(B) disposing a mixture of said media pellets and said articles in a predetermined ratio Within a movable vessel; and

(C) moving said vessel to effect relative movement between said articles until surface irregularities are removed to the extent desired.

2. A method of removing surface irregularities from hard-surface articles comprising:

(A) mixing particles of abrasive material forming interstress therebetween with a liquid w-hich freezes into a rigid solid at temperatures in the range of 0 F. to 212 F.;

(B) freezing portions of the resultant mixture into pellets of a desired size to form surface-finishing media;

(C) disposing a load comprising a mixture of said pellets and said articles in a predetermined ratio within a movable vessel; and

(D) moving said vessel to effect relative movement between said articles and said pellets until surface irregularities are removed from the article to the extent desired.

3. The method of claim 2 comprising: adding liquid to the load of articles and media in said vessel until the interstices of the load are substantially filled; and refrigerating the load to control melting of the media.

4. The method of claim 2 comprising: cooling the articles before entry into the vessel; and cooling the vessel to control melting of the media mixed with said articles.

5. The method of claim 2 comprising: after effecting desired surface finishing of the articles, melting the media 1n situ; draining and fiushing the media from the vessel; and washing said articles free of said particles before removal from the vessel.

6. The method of claim 2 comprising: removing melted media from the vessel and reforming it into more pellets.

7. The method of claim 2 comprising: providing a vessel having a hollow wall as said vessel; and cooling and heating the load of articles and media in a predetermined manner by circulating a fluid cooling medium through said wall during the step of moving the vessel, and thereafter circulating a fiuid heating medium through said Wall to melt any pellet media remaining after said step of moving the vessel.

8. The method of claim 2 wherein: the step of mixing said material and the liquid is effected by adding sufficient liquid to the material to fill the interstices thereof.

9. The method of claim 2 including: mixing particles of abrasive material of different sizes so selected for f1tting of smaller-sized particles within the interstices of larger particles; and filling the interstices remaining in' a uniform mixture of said sizes of particles with liquid and then proceeding with said freezing step.

10. A method of removing surface irregularities from hard-surface articles comprising:

(A) mixing particles of abrasive material with a liquid having a melting point less than 212 F. and greater than F.;

(B) freezing portions of the resultant mixture into pellets of a desired size to form surface-finishing media;

(C) disposing a load comprising a mixture of'pellets and said articles in a predetermined ratio within a movable vessel;

(D) moving said vessel to eect relative movement between said articles and said pellets until surface irregularities are removed from the articles to the extent desired;

(E) separating melted media and any unmelted media from said articles; and

(F) cleaning said articles by a liquid treatment to remove any of said particles therefrom.

11. The method of claim 10 including: the step of heating the articles after the separating step to melt any frozen media retained by the articles.

12. The method of claim 10 including: the step of heating the mixture of articles and media to melt all of the media after said removal of surface irregularities.

13. The method of claim 12 comprising: providing a vessel having a hollow wall as said vessel, and circulating a fluid heating medium through said wall.

14. The method of claim 10 comprising: providing a vessel having a hollow wall as said vessel, and circulating a liquid rerfrigerant through said wall to control the temperature and rate of melting of said media during the step of moving the vessel.

1S. The method of claim 10 comprising: refrigerating the load of articles and media in said vessel to control the rate of melting of the media during the step of moving the vessel.

References Cited UNITED STATES PATENTS 

1. A METHOD OF REMOVING SURFACE IRREGULARITIES FROM HARD-SURFACE ARTICLES COMPRISING: (A) FREEZING PORTIONS OF A MATERIAL WHICH COMPRISES A LIQUID WHICH FREEZES INTO A RIGID SOLID AT TEMPERATURES IN THE RANGE OF O* F. TO 212* F. INTO PELLETS OF A DESIRED SIZE AND INCLUDING ALSO WITH SAID MATERIALS, A PARTICULATE ABRASIVE MATERIAL OF GREATER HARDNESS TO FORM SURFACE-FINISHING MEDIA; (B) DISPOSING A MIXTURE OF SAID MEDIA PELLETS AND SAID ARTICLES IN A PREDETERMINED RATIO WITHIN A MOVABLE VESSEL; AND (C) MOVING SAID VESSEL TO EFFECT RELATIVE MOVEMENT BETWEEN SAID ARTICLES UNTIL SURFACE IRREGULARITIES ARE REMOVED TO THE EXTENT DESIRED. 